Inkjet printing is a nonimpact process of printing text or images by depositing inkjet ink on a print medium. The inkjet ink includes an aqueous-based ink vehicle and a colorant, such as a dye or a pigment. Dyes are more commonly used in inkjet inks because dyes are easily dissolved or solvated in the ink vehicle. However, images printed with dye-based inkjet inks are generally less waterfast, lightfast, and resistant to ozone.
In contrast, pigment-based inkjet inks provide printed images having improved waterfastness and lightfastness. While pigment-based inkjet inks have these desirable properties, the pigments are typically insoluble in the ink vehicle. The pigments form discrete particles that clump or agglomerate if they are not stabilized in the inkjet ink. To prevent the pigments from agglomerating or settling out of the ink vehicle, the pigments are uniformly dispersed in the inkjet ink and stabilized in the dispersed form until the inkjet ink is used for printing.
To form stable pigment dispersions, dispersants or surfactants are added to the ink vehicle. The dispersant is typically a polymer formed by polymerizing hydrophobic monomers, hydrophilic monomers, hydrophobic copolymers, and/or hydrophilic copolymers. The dispersant binds to a surface of the pigment, providing stability to the pigment dispersion. When printing is desired, the pigment is precipitated from the inkjet ink and onto the print medium by solvent evaporation. This process is enhanced by the presence of salts, acids, or polymers in the print medium. Inks that contain dispersants provide printed images with good durability and gloss. However, a large amount of the dispersant is needed to produce these desirable properties, which increases the viscosity of the inkjet ink prohibitively. This increase in viscosity reduces the usefulness of dispersants in inkjet inks because it affects the firing reliability and print quality of inkjet pens.
Alternatively, the pigment is stably dispersed in the inkjet ink by modifying the surface of the pigment to include covalent attachment of polymeric or organic groups. The surface-modified pigments are referred to in the art as “self-dispersed pigments” or “polymer-attached pigments.” Using surface-modified pigments improves the formulation latitude of the inkjet ink and reduces the viscosity of the inkjet ink compared to the viscosity of inkjet inks that have dispersants. However, if the inkjet ink includes unbound polymeric or organic groups, these still undesirably contribute to, and increase, the viscosity of the inkjet ink.
It would be desirable to provide an inkjet ink that produces printed images that have good gloss and durability without increasing the viscosity of the inkjet ink.